This invention relates to electrical power distribution systems either indoor or outdoor and more particularly to a power distribution system in which substantially no electromagnetic field is radiated from its transmission lines.
In a power distribution system, the high current flowing through the transmission lines radiates extremely high electromagnetic fields into the vicinity around the lines. Such high electromagnetic fields cause interferences with other electrical appliances operating nearby to the transmission lines. Furthermore, it is well known that the high electromagnetic fields radiating from the power distribution lines also may cause harmful health effect to the people living or working near such transmission lines and exposing constantly to the electromagnetic fields over a long period of time.
Heretofore, attempts have been made to shield the transmission lines in order to confine the electromagnetic fields radiating from these lines. Each transmission line, after it is installed, is provided over its entire length with an external shield layer or enclosure made of a ferromagnetic material in order to suppress the electromagnetic field. However, such method is expensive and difficult to carry out; and yet, the electromagnetic fields still exist outside of the shield.
Circulating currents in the metallic enclosures of gas insulated distribution systems and on high voltage cables, introduced by bonding such enclosures on both ends are also known to reduce the external electromagnetic field. However, these circulating currents themselves are induced to the enclosure by the external electromagnetic field, thus by virtue of their linking with the external field they cannot eliminate electromagnetic field entirely and significant external electromagnetic field is still radiated from these lines.
In U.S. Pat. No. 2,030,935 to Georg Pfaff it is shown a method of reducing the spurious induced current in the cable sheath of high voltage alternating current single core power lines. The spurious induced current generates undesirable heat in the cable sheath to cause its accelerated deterioration. In this patent a plurality of cross connections are provided between the sheath of neighbouring single core transmission lines so as to eliminate the spurious current. Such provision is therefore not primarily intended to reduce the electromagnetic field radiation from the power lines and as discussed above inherent electromagnetic field radiation still emits from the power lines. Furthermore, such method is not applicable to direct current power lines.